Silicon-on-insulator (SOI) integrated circuits include a plurality of islands of single crystalline silicon on a substrate of an insulating material, such as sapphire. Semiconductor devices, such as insulated gate field effect transistors (IGFETS) are formed in the silicon islands. Such IGFETS include a thin layer of silicon oxide over the silicon islands and gate lines of a conductive material extending across the islands and over the oxide layer. The gate lines are generally formed of conductive polycrystalline silicon, and, to make them more conductive, are often covered with a layer of a metal silicide, such as tantalum or titanium silicide. This bilayered structure is called a polycide.
To make the SOI integrated circuits, the silicon islands are first formed on the insulating substrate. The islands are then heated in an oxidizing atmosphere to form a thin layer of silicon oxide over the entire surface of the islands. A layer of polysilicon is then deposited over the entire surface of the islands and the exposed surface of the substrate. If silicide is used, it is deposited over the polysilicon layer, such as by co-sputtering the metal and silicon or by sputtering the metal and heating to convert a surface portion of the polysilicon to the silicide. The polysilicon layer, with or without the silicide layer, is then defined to form the gate lines extending over the islands. This is achieved by covering the polysilicon layer with a layer of a photoresist which is defined, using standard photolithographic techniques, to cover only the portions of the polysilicon layer which are to form the gate lines. The exposed area of the polysilicon layer is then removed leaving the gate lines. The exposed area of the polysilicon layer is generally removed by etching, usually with an anisotropic etch, such as by plasma etching.
A problem with this technique of etching the polysilicon layer is that it leaves narrow strips of the polysilicon, known as stringers, along the side walls of the silicon islands. If there are two or more gate lines being formed over a silicon island, the stringers can electrically connect such gate lines and short them to each other. To completely remove such stringers requires a long overetch. However, such long overetching is not acceptable since it will also remove the exposed silicon oxide, which is undesirable. Therefore, it would be desirable to have a plasma etching technique which would remove all of the polysilicon layer, so as to leave no stringers, but without removing the silicon oxide layer on the surface of the islands, and without undercutting the sides of the gate lines formed by the etching technique.